<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>点击事件的三种添加方式</title>
</head>
<body>
    <button id="btn1">按钮1</button>
    <button id="btn2">按钮2</button>
    <button id="btn3" onclick="demo()">按钮3</button>
    <script type="text/javascript">
        let btn1 = document.getElementById('btn1')
        btn1.addEventListener('click',()=>{
            alert('123')
        })
        let btn2 = document.getElementById('btn2')
        btn2.onclick=()=>{
            alert('456')
        }
        function demo(){
            alert('789')
        }
    </script>
</body>
</html>